Use precompiled headers to compile C++ modules

On an average system this will cut around 15% of the total build time.
Unfortunately, including Boost headers in the precompiled header takes
longer time to generate and then read in each module, than just
including the necessary headers in each module.

Use Noel Llopis' list_precomp.py at
http://www.gamesfromwithin.com/wp-content/uploads/bin/list_precomp_py.txt
to analyse which headers are included the most and are candidates for
inclusion.

opm/core/opm-core-pch.hpp

@@ -0,0 +1,10 @@
+/* List of headers that should be precompiled */
+#include <algorithm>
+#include <cstddef>
+#include <cstdlib>
+#include <fstream>
+#include <iomanip>
+#include <iostream>
+#include <istream>
+#include <string>
+#include <vector>